1. Field of the Invention
The present invention relates to a negative electrode plate for alkaline storage cells of the sealed type which is improved in oxygen gas absorbing ability, etc., and more particularly to a negative electrode plate prepared from a cadmium paste.
2. Prior Art
Negative cadmium electrode plates of the paste type have great advantages over negative cadmium electrode plates of the sintered type in that they can be fabricated at a low cost by a simple process and provide a high energy density. However, the negative electrode plate of this type is low in electron conductivity, therefore exhibits poor ability to absorb the oxygen gas evolved from the positive electrode owing to excessive charging and has the drawback that the internal gas pressure is liable to build up when it is used in sealed cells. The oxygen gas absorbing reaction at the negative cadmium electrode plate is represented by the following equation. EQU Cd+1/2O.sub.2 +H.sub.2 O.fwdarw.Cd(OH).sub.2 ( 1)
Thus, the reaction takes place at the interfaces between the three phases of gas, liquid and solid. The more metallic cadmium contacts oxygen gas, the more is the reaction accelerated. With the sintered electrode plate wherein a sintered base of nickel serves as an electrically conductive matrix, the charging reaction proceeds uniformly over the entire electrode plate through the matrix, readily permitting formation of metallic cadmium also in the vicinity of the plate surface. With the negative cadmium electrode plate of the paste type, however, the active material layer is low in electrical conductivity, so that the charging reaction slowly proceeds from the neighborhood of the carrier sheet toward the plate surface, consequently involving difficulty in forming metallic cadmium in the vicinity of the plate surface away from the conductive carrier sheet. Accordingly oxygen gas is not absorbed before passing through the plate surface layer which is low in gas permeability, hence poor ability to absorb oxygen gas.
With attention directed to the above drawback, several proposals have been made.
Proposal (1): Method of causing oxygen gas to easily penetrate into the electrode plate by giving ennanced hydrophobic tendency to the neighborhood of the plate surface. This method reduces the amount of electrolyte retained in the neighborhood of the plate surface, rendering the active material less utility and increasing the internal resistance of the cell to impair the cell characteristics.
Proposal (2): Method of facilitating presence of metallic cadmium on the plate surface by forming projections on the electrically conductive carrier sheet and thereby locally decreasing the distance between the carrier sheet and the plate surface. This method permits metallic cadmium to appear on the plate surface relatively rapidly, but it is difficult for the metallic cadmium to spread over the plate surface and to come into contact with oxygen gas over an increased area. The ability to absorb oxygen gas is therefore almost as low as that of conventional electrode plates. With cells of the sealed type, moreover, the electrode plate and separator are usually subjected to a considerable structural pressure, so that some projections on the carrier sheet are likely to become exposed at the surface of the active material layer of the soft paste-type electrode plate and to reach the positive electrode through the separator to cause shortcircuiting within the cell.
Proposal (3): Method of facilitating presence of metallic cadmium on the plate surface by forming indentations in the plate surface and locally decreasing the distance between the conductive carrier sheet and the plate surface as in the case of Proposal (2). The electrode plate thus formed is not greatly different from conventional ones in oxygen gas absorbing ability. When the plate is assembled into a cell, the resulting structural pressure is likely to collapse some indentation-defining projections on the plate surface. The indentations formed in the plate surface result in a reduction in the plate capacity, hence disadvantageous (see Published Examined Japanese Patent Application No. SHO 47-45460).
Proposal (4): Method of preforming a metal cadmium layer on the plate surface. With this method, the metallic cadmium participates directly in the charge-discharge reaction, with the result that charging and discharging, when repeated, converts the metallic cadmium in the vicinity of the plate surface to cadmium hydroxide, entailing impaired ability to absorb oxygen gas (see Published Unexamined Japanese Patent Application No. SHO 54-109143).
As already stated, therefore, the conventional negative cadmium electrode plates of the paste type for sealed alkaline cells still remain to be improved in oxygen gas absorbing ability.